Probes for the ultrasonic treatment or inspection of molten aluminum

ABSTRACT

Special probes for the ultrasonic inspection or treatment of molten aluminum are described. Such probes employ a special working tip which is made essentially of titanium and is capped with a coating of aluminum by a vacuum vaporization process. Also described is a process for making such probes.

FIELD OF THE INVENTION

This invention relates to the application of ultrasonic waves to thetreatment and inspection of molten aluminum. More particularly, itrelates to an improved probe for conducting ultrasonic waves between atransducer device and an aluminum melt.

BACKGROUND OF THE INVENTION

Ultrasonic waves have become of great importance in recent years. Theirunique properties have been applied to industry, signaling, medicine andmany other fields.

The use of ultrasonic waves to treat and inspect molten aluminum isknown, though not yet widely practiced commercially.

It is known, for example, that molten aluminum can be treated withultrasonic waves of relatively low frequencies (15-20 KHz) and highpower (0.1-several hundred watts) to achieve improvements in metalquality. The effects reported include degassing of the molten aluminumto decrease its hydrogen content, facilitated dispersion of alloyingelements in the molten aluminum, and in respect to the solidified metal,grain refinement and increased workability and mechanical properties.

For the inspection of molten aluminum, relatively high frequencies (1-10MHz) and low power (0.004-0.04 watts) are used. The most practical meansof inspection is the pulse-echo method wherein an ultrasonic wave pulseis transmitted into the molten aluminum and the pulse reflections orechoes are detected and measured. Melt quality can be characterized interms of the number and amplitude of the echoes reflected fromdiscontinuties such as insoluble melt constituents, attenuations inpulse amplitude, pulse velocity through the melt, and shifts in theultrasonic wave frequency.

Other applications of ultrasonic waves to the treatment or inspection ofmolten aluminum are of course possible.

For details concerning the implementation of ultrasonic wave technologygenerally, see B. Carlin, Ultrasonics, McGraw-Hill Book Company, Inc.,New York-Toronto-London (1960), the disclosure of which is herebyincorporated herein by reference.

To transmit or receive ultrasonic waves to or from an aluminum melt, itis common to use an electromechanical transducer device for convertingelectrical energy to mechanical energy and vice versa. The most popularelectromechanical conversion systems rely either on magnetostriction orthe piezoelectric effect to operate. However, magnetostrictivetransducers are not generally used for inspecting molten aluminumbecause of their characteristic low operating frequency (e.g. 60 KHz orless).

Piezoelectric transducers typically have the capability to both transmitand receive ultrasonic waves. Thus a single piezoelectric transducer maybe used to perform both functions, or separate transducers may be usedfor transmitting and receiving. Piezoelectric transducers can readily bemade to handle high frequencies and low power levels, and areaccordingly well suited for molten aluminum inspection methods.

A transducer can conveniently be coupled to the melt using a probe,sometimes called a "delay line" or a "mechanical standoff". See, forexample, U.S. Pat. No. 3,444,726 to R. S. Young et al. The probe servesto isolate the transducer from the high melt temperatures, which willusually run in the range of about 675° to 825° C., and to introduce atime delay between a transmitted pulse and echoes from inclusionslocated near where the pulse first enters the melt.

The probe will usually be in the form of a bar or rod, one end of whichwill be immersed in the melt and is known as the "working tip". And theother probe end is coupled to the transducer. It has been said that anideal probe material should have the following properties:

(a) The material should have a constant low acoustic energy attenuationover the range of working temperatures at the frequencies used.

(b) It should be sound and homogeneous and have good resistance tothermal and mechanical shock.

(c) It should have a good resistance to attack by the molten metal. Anymaterial which has the effect of reacting with the molten metal to forma protective film has the disadvantage that wetting of the immersedtransmitting end of the probe by the molten metal will be materiallyreduced.

(d) It should have a low thermal conductivity.

(e) The acoustic impedance i.e. the product of density and the velocityof sound, should be of the same order as of the molten metal.

Apparently no material has been found which would fulfill all of theserequirements.

Sintered rods made from titanium diboride and titanium carbide mixturesin 70/30 and 60/40 volumetric proportions have, for example, beenexamined by the prior art. With these rods, difficulty was encounteredinitially in obtaining rods of adequate soundness and in wetting theimmersed ends of the rods to allow transmission of ultrasonic energybetween the liquid aluminum and the probes. In attempts to effectwetting, the probes were immersed in the liquid aluminum under an inertatmosphere or argon. These attempts were not successful, even when theprobe ends were capped with brazing metal before immersion. Greatersuccess was obtained when the rods were capped with pure aluminum athigh temperatures (e.g. 1200° C.) under vacuum; these gave lowattenuation and a very small loss of signal at the probe-aluminuminterfaces. However, these benefits were lost when the probes wereremoved from the liquid metal and exposed to atmosphere. The probe endsurfaces apparently oxidized so that on reimmersion full wetting did notoccur and only a small proportion of the available signal was thentransmitted into the metal.

A titanium alloy, Ti 317, containing 5% Al and 2.5% Sn (by weight) andobtainable with a single phase structure, was also examined by the priorart and found to resist erosion to a considerable extent. Materialhaving a duplex (α+B) structure had a very high attenuation, so that itwas only possible to transmit signals up to 2.5 MHz through a 2 ft.×1in. diameter rod. When converted to a single phase structure, it had areasonable attenuation, though still higher than desirable. Also,experiments show that titanium does not become wetted until it has beenimmersed in molten aluminum for approximately thirty minutes.

After looking at titanium diboride-titanium carbide sinters and metallictitanium alloys as probe materials, at least one group came to prefersteel (0.26 wt.% carbon content) coated with a sprayed water-suspendedFoseco Dycote 34 and tipped with a cap of silver solder. The silversolder accelerated the wetting so that the probes transmitted andreceived the available energy after approximately three minutesimmersion. Once wetted, the probes could be removed from the liquidmetal, allowed to cool and then replaced without undue loss of couplingefficiency. And the sprayed refractory coating prevented wetting of thesides of the probes and the introduction of stray vibrations into theliquid metal. It was a problem, however, that the steel tended to bedissolved in the aluminum melt. The probelm was tolerated by observingthe amplitude of the reflected echoes, and when the amplitude fell to apredetermined level, the probes were removed, shortened and resoldered.

Hence, of various probe constructions that the prior art had looked at,each was effected by one or more of the following problems: wetting didnot occur at all or only until the passage of some substantial amount oftime after the probe was initially immersed in the melt; wetting did notoccur after the probe was removed from the melt, exposed to theatmosphere and cooled, and then re-immersed; at operating temperatures,the probe material attenuated the ultrasonic signals to an undesirabledegree; or the probe material was not chemically stable in moltenaluminum.

It was against the foregoing background that this invention was made.

INVENTION SUMMARY

This invention is directed to improved probes for conducting ultrasonicmechanical energy between a transducer device and an aluminum melt. Aprobe as improved herein comprises a working tip which is madeessentially of titanium, exhibiting preferably a single phase structure,and a coating of aluminum which has been volatilized and desposited onthe working tip in a vacuum.

For a probe to transmit or receive ultrasonic waves to or from moltenaluminum, its working tip must be "wetted" by the molten metal. Theimproved probe according to this invention becomes wetted very quicklyat its working tip; and the probe is characterized in that uponimmersion of the tip in an aluminum melt at temperatures up to 850° C.,the tip is wetted by molten aluminum in about one minute or less,usually is about 15 seconds. And once wetted, the probe working tip canbe removed from the liquid metal, exposed to the atmosphere and allowedto cool, and then re-immersed in the melt, with re-wetting occurring ina similarly short time, e.g. in about one minute or less, and usually inabout 15 seconds.

The attenuation of ultrasonic energy within the probe is typically not aproblem for high power applications (0.1 watts or more).

For low power applications (0.004-0.04 watts), it is desirable to have aworking tip which is short (e.g. 1/4 to 2 inches in length), and coolingmeans to ensure a steep negative temperature gradient (e.g. at least200° C./in) along the longitudinal probe axis from the point ofimmersion to a point where the probe temperature is reduced to 300° C.or less.

The improved probe according to this invention has proved to besubstantially inert with respect to molten aluminum.

This invention also embraces a special process for producing an improvedprobe as described above. Starting with a probe working tip which ismade essentially of titanium, the tip is chemically etched to clean andremove titanium oxides and other reaction products from the tip surface.The chemically etched tip is then situated in a vacuum atmosphere whereit is bombarded with an ionized gas from a glow discharge to effect afurther cleaning and titanium oxide and other reaction products removal.The pressure of the vacuum atmosphere is then decreased, and aluminum isvolatilized in the vacuum atmosphere of decreased pressure so that thevolatilized aluminum is deposited on the tip to form a coating thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings:

FIG. 1 is a side view of a special molten aluminum inspection probedescribed herein;

FIG. 2 is a front view of the probe shown in FIG. 1, with sectionsremoved and sections broken away; and

FIG. 3 is an exemplary circuit block diagram for operating the probeshown in FIGS. 1 and 2.

DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

FIGS. 2 and 3 illustrate an improved probe 1 which is a presentlypreferred embodiment of this invention. It will be understood that theprobe 1 is particularly suited and adapted for molten aluminuminspection methods, and that for other applications such as thetreatment of molten aluminum, it may be necessary to provide adaptationsthat will be obvious and within the ordinary skill in the art.

The probe 1 includes and comprises a body member 3 which is madeessentially of titanium, and is preferably machined from a wroughttitanium bar having a single phase structure. The body member 3 hasopposite ends 5 and 7, each of which has a substantially flat endsurface 9 which is substantially perpendicular to the longitudinal axis11 of the body member 3.

A suitable piezoelectric transducer device 13 may be convenientlycoupled to the surface 9 of body member end 7 by mechanical pressureapplied against an O-ring 15 by a hold-down plate 17 as illustrated inFIG. 2. As shown, the plate 17 may be secured by a plurality ofthumbscrews 19. To minimize acoustical resistance, a suitable couplingmedium, e.g. a foil shim or a suitable high temperature oil or grease,should be interposed between the surface 9 of the body member end 7 andthe working contact surface of the transducer device 13. For example,the coupling medium may consist of a suitable silicone compound such asDow Corning Corporations's 710 silicone fluid (serviceable from 0° to500° F.).

The transducer device 13 may be of any conventional type which operateswithin the desired ranges for frequency and power. Transducers whichemploy a crystal made of quartz, barium titanate or a suitable ceramicmaterial may generally be used with the illustrated embodiment of thisinvention. For example, A-3000 series flat immersion type search unitsmade by Panametrics, Inc. may be used.

Operability of a typical piezoelectric transducer is limited by itsCurie point temperature. A transducer which uses a quartz crystal mustusually be operated at temperatures of 300° C. or less, for example. Atransducer which uses a barium titanate crystal must usually be operatedat temperatures of 110° C. or less. Thus it will be important to coolthe probe 1 such that the surface 9 of body member end 7 will have atemperature within the operating limit for the transducer device 13. Thetemperature of this surface 9 can be monitored using a thermocoupleconnected at hole 21 shown in FIG. 2.

The probe 1 has a working tip which is formed by the end 5 of the bodymember 3. To avoid undue attenuation of ultrasonic signals by thetitanium probe material, the working tip should be relatively short.Thus, the working tip should have a length A which is about 1/8 to 3inches, preferably about 1/4 to 2 inches, e.g. about 3/8 inches.

As shown, the body member 3 is defined in part by a lateral portion 29adjacent the working tip, where the lateral portion 29 has a length Band defines a cooling zone for the probe 1. The length B should berelatively short, preferably about 21/2 to 3 inches, to avoid undueattenuation of the ultrasonic signals while allowing sufficient surfacearea for the probe 1 to be adequately cooled.

Heat is extracted at the above-mentioned cooling zone by cooling meanssuch as a water jacket 31 or the like. The water jacket 31, which may bemade of brass and shrink fitted in place, should have the capacity tocool the probe 1 in a manner such that when its working tip reaches athermal equilibrium upon being immersed in an aluminum melt at atemperature in the range of about 675° to 825° C., there exists anegative temperature gradient of at lest 200° C./in, .e.g. about 250°C./in, along the axis 11 within the probe cooling zone. This negativetemperature gradient should reduce the probe temperature along the axis11 from the point of immersion, e.g. point C, to a point where the probetemperature is 300° C. or less. This negative temperature gradient isnecessary to avoid undue attenuation of low power ultrasonic signals bythe titanium probe material. It should also be effective to reduce thetemperature of the surface 9 of body member end 7 within the operatinglimit, for the piezoelectric transducer device 13.

As shown in FIG. 2, the water jacket 31 may comprise a two pieceassembly including a cap 22 which can be secured with silver solder. Inthe figure, the thumbscrews 19 pass through the transducer hold-downplate 17 and are threaded into the cap 22. As security against slippagebetween the probe lateral portion 29 and the water jacket 31 when theprobe 1 is operated, an additional thumbscrew (not shown) may be passedthrough the hold-down plate 17 and threaded into the body member end 7.

The probe 1 may also include a substantially flat reflective surface 23is opposed, spaced apart, substantially parallel relationship within thesurface 9 of the end 5 of body member 3. As shown in FIG. 2, thereflective surface 23 may be spaced from the surface 9 by a pair ofopposed support walls 25. Thus, the surface 9 of the end 5 defines acavity 27 which is further defined by the support walls 25 and thereflective surface 23. The cavity 5 will fill with molten metal when theprobe 1 working tip is immersed, and it will accommodate a molten metalflow when the working tip is immersed for inspecting a molten aluminumstream. If desired, the support walls 25, the reflective surface 23 andthe body member 3 may all be made using a single piece of titanium barstock in which the cavity 5 may be formed using a conventional machiningoperation.

The surface 9 of the end 5 and the reflective surface 23 may eachmeasure about 2 inches by 2 inches square and are spaced apart by adistance D such that in operation ultrasonic signals transmitted fromthe surface 9 are reflected off of the reflective surface 23 and back tothe surface 9 of end 5 after traveling a known reference distance. It isnecessary to use a reflective surface, such as the surface 23, forpurposes of detecting and evaluating velocity and comparative amplitudeattenuation and frequency shifts of the ultrasonic signals. A referencereflective surface is not needed for discontinuity detection, however.It would, of course, be possible to omit the reflective surface 23 alongwith the support walls 25 and to utilize a suitable substitutereflective surface detached from the probe 1 and positioned within thealuminum melt. Part of the structure which contains the melt might beutilized, for example. However, the illustrated configuration greatlysimplifies things by allowing the probe 1 to be installed and movedwithout having to worry about aligning the probe 1 or re-establishingthe distance that the signals will travel.

The reflective surface 23 should be spaced at least about 1/2 inch fromthe surface 9 of the probe end 5. For example, a spacing distance D ofabout 11/2 to about 2 inches can be used. This assures a signal paththrough the melt that is of sufficient length so that characteristicdata can be obtained. The spacing distance D should also be such thatthe ratio of the distance D to the probe length (A+B) is less than theratio of the signal velocity through the melt to the average signalvelocity through the probe (under operating conditions). This is toavoid the possibility of an overlap between the received signals fromthe melt and the second received reflection from the probe-meltinterface.

According to this invention, the probe working tip is capped with acoating of aluminum which has been volatilized and deposited on theworking tip in a vacuum. And the probe 1 is characterized in that uponthe immersion of the working tip in an aluminum melt at temperatures upto about 850° C., the working tip is wetted by molten aluminum in aboutone minute or less, e.g. about 15 seconds. Once it is wetted, the probeworking tip can be removed from the liquid metal, exposed to theatmosphere and allowed to cool, and then re-immersed in the melt withre-wetting occurring in a similarly short time, e.g. in about one minuteor less, and usually in about 15 seconds.

The aluminum coating can conveniently be applied to the probe workingtip using the following special process.

First, the working tip is chemically etched to clean and to removetitanium oxides and other reaction products from the working tipsurface. This step may be carried out using a suitable acidic aqueoussolution containing at least one acid selected from the group consistingof chromic acid, hydrofluoric acid, phosphoric acid, nitric acid, sodiumsulfate and sulfuric acid. Satisfactory results have been obtained, forexample, using a solution which consists essentially of about 20 wt.%hydrofluoric acid and about 30 wt.% nitric acid, balance water.Preferably, the etching step is continued until a sufficient amount ofelemental titanium is removed from the probe working tip for theunderlying titanium grain structure to become visible at the working tipsurface.

The etched workpiece is then situated in a vacuum atmosphere preferablyof about 50 to 300 microns pressure, e.g. about 200 microns pressure,where the working tip surface is bombarded with ionized gas from a glowdischarge preferably for a period of about 15 to 60 minutes, e.g. about45 minutes. These steps further clean and remove titanium oxides andother reaction products from the working tip surface.

The vacuum atmosphere pressure is decreased preferably to about 0.005 to0.5 microns pressure, e.g. about 0.01 microns pressure; and thenaluminum is volatilized in the presence of the workpiece preferably forabout 15 to 30 seconds, e.g. about 20 seconds, such that the volatilizedaluminum is deposited on the working tip surface to form the desiredcoating.

To seal the coated probe working tip and to inhibit its oxidation, it isgood practice to immerse the working tip in an aluminum melt within afew minutes after the probe is removed from the vacuum atmosphere. Itmay also be helpful to operate the probe at this time. Upon removing theprobe from the melt, the probe may be allowed to cool and then stored.

The probe 1 can be operated using conventional pulse-echo circuitrywhich is well known in the art. U.S. Pat. No. 2,280,226 to F. A.Firestone discloses circuitry for a reflectoscope that can be used forexample. A Model S-80 reflectoscope with a Model PR-1 pulser/receivermade by Automation Industries, Inc. can be used with good results.Alternatively, a Model 9000 attenuation comparator made by Matec, Inc.could be used.

An exemplary circuit block diagram for operating the probe 1 isillustrated in FIG. 3. As shown, outputs from a pulse oscillator 33 anda high frequency oscillator 35 are supplied to a modulator 37 which inturn supplies an output to amplifier 39. The resultant amplifier 39output is a radio-frequency pulse of a few microseconds in duration at arepetition rate of between about 50 Hz to about 5000 Hz, e.g. about 2.5KHz. The repetition rate of this r.f. pulse is not critical, but itshould be sufficiently slow to prevent reflections from successivepulses from overlapping each other. The maximum pulse amplitude may beon the order of a few hundred to several thousand volts, but raising thevoltage does not necessarily raise the sensitivity proportionately, andabout 500 volts will work very well. The pulse carrier frequency shouldbe on the order of what is required for molten aluminum inspectionmethods, e.g. 9.5 MHz, and will, of course, depend on the operatingcharacteristics of the piezoelectric transducer 13.

The amplifier 39 output is supplied to the transducer 13 through animpedance matching network 41 which matches the transducer capacitancewith an inductance for improved operating efficiency. In response to theamplifier 39 output, the transducer 13 sends out ultrasonic signalsthrough the probe 1 and into the melt under inspection. Reflections ofechoes of these signals are returned to the transducer 13. Thetransducer 13 converts the echoes to electrical energy which is suppliedback through the matching network 41 to tuned receiver 43. The receiver43 output is in turn supplied to an oscilloscope 45 through a variablesignal attenuator 49. The oscilloscope 45 is synchronized by aconnection of its linear sweep 51 to the pulse oscillator 33 through avariable delay 53.

Since the transmitted and the received pulses are both impressed uponthe receiver 43, they are both readable simultaneously on the visualdisplay of the oscilloscope 45. However, the delay 53 may be adjusted sothat the transmitted pulses do not appear.

The attenuator 49 may be adjusted so as to regulate the amplitude of thedisplayed pulses and is useful for purposes of calibration. For example,when inspecting for discontinuities or insoluble particulate matter inthe melt, it is useful for calibrating the display according to adistanceamplitude-correction curve for a known type and size defect suchas a 1/4 inch alumina ball.

Read out sensitivity for the received pulses can be adjusted byselection or adjustment of the transmitted pulse carrier frequency.Since sensitivity may vary according to the type of measurement that isbeing made, it is convenient to provide a variable control feature forthe high frequency oscillator 35.

The probe 1 as thus described is a device which is well suited formonitoring and establishing the quality of molten aluminum in metalcleaning and casting operations. The device is simple, rugged, reliableand is adapted for use in an aluminum cast house on a daily, routinebasis as a production quality control tool. It may be used to learnabout or demonstrate the effect of some factor or variable in a process,to establish a quality level requirement for a particular process andproduct, or to compare quality in a given operation to previouslyestablished quality level criteria.

Using the probe 1, metal quality can be measured ultrasonically withrespect to discrete particulate, with respect to changes in signalattenuation, and with respect to quality problems related to velocitychanges or frequency changes. With the appropriate electronics andread-out devices, any of these quality measurements can be made singly,or in any combination desired, with a single probe in the molten metal.

As mentioned in reference to FIG. 3, the instantaneous state of qualityof the metal in the probe cavity can be known through a display of themeasuring characteristic or characteristics on a cathode ray tube.Alternatively or simultaneously, the signal or signals may be recorded.The recording can serve as an integrated measure of quality for thewhole quanity of metal which flows through the probe, with any variationor problem of quality becoming known on a time scale extending for acomplete operation.

To measure, or record metal quality at several locations, multipleprobes can be used, or a single probe can be shifted among locationssince there is no appreciable delay in the functioning of a probe eitherwhen it is first put into use or when it is shifted from one position toanother.

While this invention has been described with reference to a probe whichperforms both functions of transmitting and receiving ultrasonicsignals, it will be understood that the invention is equally applicableto probes intended for carrying out only one of those functions. Andwhile the invention relates to probes which use a piezoelectrictransducer for practicing methods of molten aluminum inspection, it alsorelates to probes which may be coupled with other types of transducers,e.g. magnetostrictive transducers, and to probes which may be used forpracticing methods of molten aluminum treatment. Thus it will berecognized that numerous embodiments of the invention are possible, itbeing intended that the invention be defined and limited only by thescope of the following claims.

What is claimed is:
 1. An improved probe for conducting ultrasonicmechanical energy between a transducer device and an aluminum melt;where said probe comprises a working tip consisting of titanium, and acoating of aluminum which has been volatilized and deposited on saidworking tip in a vacuum; and said probe is further characterized in thatupon the immersion of said working tip in an aluminum melt attemperatures up to about 850° C., said working tip is wetted by moltenaluminum in about one minute or less.
 2. An improved probe forconducting ultrasonic mechanical energy between a transducer device andan aluminum melt; where said probe comprises a working tip madeessentially of titanium, and a coating of aluminum wherein said coatinghas been formed on said working tip by sequential steps comprising:(a)chemical etching the surface of said working tip to clean said surface,and to remove titanium oxides and other reaction products from saidsurface; (b) situating said working tip in a vacuum atmosphere; (c)bombarding the surface of said working tip with ionized gas from a glowdischarge to further clean said surface, and to further remove titaniumoxides and other reaction products from said surface; (d) decreasing thepressure of said vacuum atmosphere; and (e) volatilizing aluminum insaid vacuum atmosphere so that the volatilized aluminum is deposited onthe surface of said working tip to form said coating.
 3. An improvedprobe according to claim 2 wherein step (a) is carried out using anacidic aqueous solution containing at least one acid selected from thegroup consisting of chromic acid, hydrofluoric acid, phosphoric acid,nitric acid, sodium sulfate and sulfuric acid.
 4. An improved probeaccording to claim 3 wherein said solution consists essentially of about20 wt.% hydrofluoric acid, and about 30 wt.% nitric acid, balance water.5. An improved probe according to claim 3 wherein step (a) removes asufficient amount of elemental titanium from the surface of said workingtip for the titanium grain structure of said body member to becomevisible at the surface of said working tip.
 6. An improved probeaccording to claim 2 wherein step (c) is carried out in a vacuumatmosphere of about 50 to 300 microns pressure.
 7. An improved probeaccording to claim 6 wherein step (c) is carried out for a period ofabout 15 to 60 minutes.
 8. An improved probe according to claim 2wherein step (e) is carried out in a vacuum atmosphere of about 0.005 to0.5 microns pressure.
 9. An improved probe according to claim 8 whereinstep (e) is carried out for a period of about 15 to 30 seconds.
 10. Animproved probe according to claim 2 wherein following step (e), thecoated working tip is removed from said vacuum atmosphere and sealed toinhibit its oxidation by immersing said coated working tip in analuminum melt.
 11. A process of making a probe for conducting ultrasonicmechanical energy between a transducer device and an aluminum melt,where said process comprises the following sequential steps, startingwith a probe working tip which is made essentially of titanium;(a)Chemical etching the surface of said working tip to clean said surface,and to remove titanium oxides and other reaction products from saidsurface; (b) Situating said working tip in a vacuum atmosphere; (c)Bombarding the surface of said working tip with ionized gas from a glowdischarge to further clean said surface, and to further remove titaniumoxides and other reaction products from said surface; (d) Decreasing thepressure of said vacuum atmosphere; and (e) Volatilizing aluminum insaid vacuum atmosphere so that the volatilized aluminum is deposited onthe surface of said working tip to form a coating thereon.
 12. A processaccording to claim 11 wherein step (a) is carried out using an acidicaqueous solution containing at least one acid from the group consistingof chromic acid, hydrofluoric acid, phosphoric acid, nitric acid, sodiumsulfate and sulfuric acid.
 13. A process according to claim 12 whereinsaid solution consists essentially of about 20 wt.% hydrofluoric acidand about 30 wt.% nitric acid, balance water.
 14. A process according toclaim 12 wherein step (a) removes a sufficient amount of elementaltitanium from the surface of said body member for the titanium grainstructure of said body member to become visible at the surface of saidbody member.
 15. A process according to claim 11 wherein step (c) iscarried out in a vacuum atmosphere of about 50 to 300 microns pressure.16. A process according to claim 15 wherein step (c) is carried out fora period of about 15 to 60 minutes.
 17. A process according to claim 11wherein step (e) is carried out in a vacuum atmosphere of about 0.005 to0.5 microns pressure.
 18. A process according to claim 17 wherein step(e) is carried out for a period of about 15 to 30 seconds.
 19. A processaccording to claim 11 wherein an improved probe according to claim 2wherein following step (e), the coated working tip is removed from saidvacuum atmosphere and sealed to inhibit its oxidation by immersing saidcoated working tip in an aluminum melt.